This invention relates to the art of wet-pelletizing carbon black. More particularly, this invention relates to an improvement in an apparats which is particularly adapted to carrying out the pelletizing process.
Carbon black is produced by the dissociation or cracking of hydrocarbons by the thermal, impingement, or furnace method. The product produced by any of the processes is characteristically an extremely light, finely-divided, fluffy, bulky material.
It has long been known that the apparent density of the black can be advantageously increased by conversion of the black to comparatively small, generally spherical, and relatively dense beads. Such beads or pellets are sufficiently stable so that they can be shipped and ultimately used in a dustless condition. The pellets are nontheless sufficiently friable that they can be satisfactorily broken-up in a typical compounding process.
A typical process for treating the carbon black to form the spheres or pellets is by agitating a mixture of the carbon black in a suitable wetting agent, as for example, water, in a stationary vessel whose configuration is advantageously an elongated cylinder. The agitating action is obtained by rotating within the cylindrical vessel an axially positioned shaft having a plurality of spaced pins radially extended therefrom. In such pin pelletizers the length of the individual pins is such to provide a clearance of from 4 to about 90 mm. between the inner surface of the cylinder and the tip of the pins. The pins are advantageously arranged in one or more helical flights.
A disadvantage of the previously described pug-mill pelletizers is the accelerated wear on the tip of the pin from the abrasive action of the carbon black which builds up on the tubular housing filling the space between the housing and the tip of the pins. Pins must be replaced when several have broken or when the clearance between the pin tip and the inner surface of the cylinder becomes sufficiently large from pin wear to permit a thick build-up of carbon black on the inner surface of the cylinder. Friction between the built-up cake of black and the pin tips necessitates the use of additional power, beyond that required to cause pellet formation, to rotate the shaft. Additionally, chunks of cake occasionally break away from the build-up of cake on the housing, momentarily imposing exceptionally heavy loads on the drive and causing power surges. One method to minimize the formation of accretions has been disclosed in U.S. Pat. No. 3,607,086. By the method of that patent, water at about the boiling point is circulated through a water jacket mounted on the cylindrical housing of the mill.